Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple Input Multiple Output (MIMO) communication systems, such as Digital Subscriber Line (DSL) communication systems.
As the demand for higher data rates increases, DSL systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say, transmission lines that are in close vicinity such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).
A MIMO system can be described by the following linear model:Y(f)=H(f)X(f)+Z(f)  (1)wherein the N-component complex vector X, respectively Y, denotes a discrete frequency representation of the symbols transmitted over, respectively received from, the N channels, wherein the N×N complex matrix H is referred to as the channel matrix: the (i,j)-th component of the channel matrix H describes how the communication system produces a signal on the i-th channel output in response to a symbol being transmitted to the j-th channel input; the diagonal elements of the channel matrix describe direct channel coupling, and the off-diagonal elements of the channel matrix describe inter-channel coupling (also referred to as the crosstalk coefficients),and wherein the N-component complex vector Z denotes additional noise present over the N channels, such as alien interference, thermal noise and Radio Frequency Interference (RFI).
Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (or vectoring).
One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoding matrix before being transmitted over the respective communication channels. The precoding matrix is such that the concatenation of the precoder and the communication channel results in little or no interference at the receiver. This is achieved by adding to the original signal an anti-phase signal that is the inverse of an estimate of the aggregate crosstalk signal.
A further technique for reducing inter-channel interference is joint signal post-processing: the received data symbols are jointly passed through a crosstalk cancellation matrix before being detected. The crosstalk cancellation matrix is such that the concatenation of the post-coder and the communication channel results in little or no interference at the receiver. This is achieved by subtracting from the received signal an estimate of the aggregate crosstalk signal.
Signal vectoring is typically performed at a traffic aggregation point, whereat all the data symbols that are to be concurrently transmitted and/or received are available. Signal precoding is particularly suited for downstream communication, while signal post-processing is particularly suited for upstream communication.
The choice of the vectoring group, that is to say the set of communication lines, the signals of which are jointly processed, is rather critical for achieving good crosstalk cancellation performances. Within that group, each communication line is considered as a disturbing line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line receiving crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.
Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interact with each other. Yet, limited vectoring capabilities and/or specific network topologies may prevent such an exhaustive approach, in which case the vectoring group would include a sub-set only of all the physically interacting lines, thereby yielding limited crosstalk cancellation performances.
The performance of signal precoding and post-processing depends critically on the component values of the precoding and cancellation matrix respectively, which component values are to be computed and updated according to the actual (and varying) crosstalk coupling functions between the respective communication channels.
A prior art method for estimating the crosstalk coefficients comprises the steps of:                simultaneously transmitting a plurality of mutually orthogonal crosstalk pilot sequences of length L through respective ones of a plurality of disturber channels,        measuring errors induced over a victim channel while the pilot sequences are being transmitted,        correlating the error measurements with respective ones of the plurality of crosstalk pilot sequences, thereby yielding a plurality of correlated error measurements,        estimating the crosstalk coefficients from the plurality of disturber channels into the victim channel based on respective ones of the plurality of correlated error measurements.        
That is, transceiver units send mutually orthogonal downstream and/or upstream pilot signals. Error samples, measuring both interference and noise over the victim channel, are fed back to a Vectoring Control Entity (VCE). Error samples contain both amplitude and phase information on a per-tone basis, or on a per-group-of-tones basis. The error samples are correlated with a given pilot sequence in order to obtain the crosstalk contribution from a specific line. To reject the crosstalk contribution from the other lines, i.e. in order to fulfill the orthogonality requirement, a multiple of L error samples shall be collected and processed. The crosstalk estimates are used for updating the precoding and/or cancellation matrix. The process can be repeated as needed to obtain more and more accurate estimates.
The orthogonality requirement further implies that the length L of the pilot sequences is lower-bounded by the size of the vectoring group: the more channels, the more lengthy the pilot sequences, the longer the estimation of the crosstalk coefficients.
This prior art method has been adopted by the International Telecommunication Union (ITU) for use with VDSL2 transceivers, and is described in the recommendation entitled “Self-FEXT Cancellation (vectoring) For Use with VDSL2 Transceivers”, ref. G.993.5 (April 2010).
In this recommendation, it is currently envisaged that the pilot signals would be sent on the so-called SYNC symbols, Which occur periodically after every 256 DATA symbols.
On a given disturber line, a representative subset of the active carriers (or tones) of the SYNC symbol are 4-QAM modulated by the same pilot digit (+1 or −1) from a given pilot sequence, and thus all transmit one of two complex constellation points, either ‘1+j’ corresponding to ‘+1’, or ‘−1−j’ corresponding to ‘−1’. The remaining carriers of the SYNC symbol keeps on carrying the typical SYNC-FLAG for EOC message acknowledgment.
On a given victim line, error samples are measured and reported for a specific SYNC symbol to the VCE for further crosstalk estimation.
In G.993.5, it is further assumed that the access node transmits and receives the SYNC symbols over the vectored lines synchronously (super frame alignment) so as pilot signal transmission and error measurements occur simultaneously.
If a line comes into service (e.g., after modem start-up at subscriber premises), the crosstalk coefficients from the new joining line into the already active lines need to be estimated first, and the precoder and/or post-coder be updated accordingly, before the new joining line can transmit at full power over the DATA symbols, else the raising interference may bring about a line retrain on a few active lines (if the newly induced interference exceeds the configured noise margin). Similarly, the crosstalk coefficients from the already active lines into the joining line need to be estimated first, and the precoder and/or post-coder be updated accordingly, before the new joining line starts determining respective carrier bit loadings and gains so as to take full profit from the vectoring gains.
G.993.5 defines new crosstalk acquisition phases during the VDSL2 initialization procedure for acquiring the crosstalk coefficients from the new joining line into the active lines, and vice-versa.
A first crosstalk acquisition phase is carried out after the HANDSHAKE phase, whereby peer transceiver units acknowledges their mutual presence, exchange their respective capabilities and agree on a common mode of operation, and the CHANNEL DISCOVERY phase, during which peer transceiver units exchange basic communication parameters through the SOC channel while transmitting at full power within the assigned communication band. The first crosstalk acquisition phase is termed O-P-VECTOR 1 and R-P-VECTOR 1 for downstream and upstream communication respectively, and aims at estimating the downstream and upstream crosstalk coefficients from the initializing line into the already active lines. O-P-VECTOR 1 and R-P-VECTOR 1 signals comprise SYNC symbols only, which are aligned with the SYNC symbols of the active lines, and thus do not impair communication over the active lines. O-P-VECTOR 1 is followed by O-P-VECTOR 1-1; R-P-VECTOR 1 is followed by R-P-VECTOR 1-1 and R-P-VECTOR 1-2.
A second crosstalk acquisition phase is carried out after the CHANNEL TRAINING phase takes place, that is to say after the time equalizer and/or the echo canceler have been adjusted, and before the CHANNEL ANALYSIS AND EXCHANGE phase, that is to say before signal to Noise and Interference Ratio (SNIR) is measured and corresponding bit loading and gain values are determined for the respective carriers. The second crosstalk acquisition phase is termed O-P-VECTOR 2-1 and R-P-VECTOR 2 for downstream and upstream communication respectively, and aims at estimating the crosstalk coefficients from the already active lines into the initializing line.
A clause in §10.3 of G.993.5 ITU recommendation states that “if several lines are initialized simultaneously, the initialization procedures of these lines have to be aligned in time, so that all lines pass the vectoring-related phases simultaneously (see clauses 10.3.3.6 and 10.4.3.9)”. Further in §10.3.3.6 op. cit., the following further technical details are mentioned in case multiple lines are initialized:                “The downstream crosstalk channels from the initializing lines into the active lines of the vector group should be estimated simultaneously by insuring that O-P-VECTOR 1 signals are sent on all initialization lines during the estimation. This can be done by controlling the end and the start of O-P-VECTOR 1 in each line.        The upstream crosstalk channels between the initializing lines and the active lines of the vector group should be estimated simultaneously by insuring that R-P-VECTOR 1 signals are sent on all initialization lines during the estimation. This can be done by controlling the end of R-P-VECTOR 1 with the O-P-SYNCHRO V1 signal in each line.”        
One option would be to require that lines in a vectoring group are always activated sequentially. However, this may lead to a denial of service for any further lines that want to join after a single line is being initialized.
The contribution entitled “G.vector: Facilitating simultaneous initialization of multiple joining lines” from Alcatel-Lucent, and submitted during the ITU session that took place in Atlanta in February 2009, ref. 09AG-044, proposes the following algorithm for optimizing simultaneous joining of multiple lines:                “The VCE always maintains a queue of lines that have requested joining, but have not yet joined.        When ready, e.g., when a previous set of lines have finished joining, the VCE starts initialization for all lines in the queue (thus emptying the queue).        Initialization proceeds independently until it reaches the O-P-VECTOR 1 state. Each line will stay in the O-P-VECTOR 1 state until receiving OP-SYNCHRO V1 signal from its respective VTU-O (under VCE control). The VCE maintains lines in O-P-VECTOR 1 until all crosstalk coefficients targeted by this estimation stage, on all lines, have been estimated with sufficient accuracy. Then VCE sends release signal allowing all lines in the group to exit O-P-VECTOR 1, and enter CHANNEL-DISCOVERY.”        
This prior art method is disadvantageous in that the transceiver units shall remain in O-P-VECTOR 1 state till all simultaneously joining lines reaches O-P-VECTOR 1. Then, a full pilot sequence transmission shall take place, meaning at least L further SYNC symbols shall be awaited, to acquire the very first crosstalk estimates. Nonetheless, the O-P-VECTOR 1 phase is upper-bounded in time by G.993.5 recommendation: O-P-VECTOR 1 shall be less than 1024 SYNC symbols long, meaning 1024×257×250×10−6≈65s. Queuing the lines in this phase directly impacts the amount of remaining time for crosstalk estimation, and thus the accuracy of the crosstalk estimation process, and eventually the performances of vectoring.